Afaik currently you need to copy the url of the post, open it in your browser and then download the image. The less elegant solution would be to take a screenshot and crop it down.
Ugh, I was afraid of that. I’m also testing Memmy, and after some prodding I managed to get its image viewer to present a save icon when tapping on images from the post view, so I can do it there for now, but I’m sure not everyone runs two clients.
I’m really used to a long press popping up a context menu in addition to whatever other UI there may be, so hopefully at some point one of the apps will add that, and Mlem will add save in some fashion.
(I love to keep cute cat pictures. Missing mine a lot!)